Thermostatic switch with permanent magnet induced snap action



Sept. 21, 1965 A. J. CHINN 3,207,877

THERMOSTATIC SWITCH WITH PERMANENT MAGNET INDUCED SNAP ACTION Original Filed Feb. 24, 1960 't ll 14/977710? JOf/A/ OWN/V Q wand; JW Mm Ar J7 United States Patent 1960. This application May 2, 1963, Ser. No. 282,829 Claims priority, application Great Britain, Mar. 2, 1959, 7,229/59 9 Claims. (Cl. 200-138) This application is a continuation of my copending application Serial No. 10,693, filed on February 24, 1960, and now abandoned.

This invention concerns improvements in magnetic snapaction switches of the type in which a U-shaped armature embraces a permanent magnet.

In contradistinction to previous switches of this type, according to the present invention, there is provided a snap-action switch wherein a U-shaped or bifurcated armature embraces a permanent magnet, such armature being carried by a resilient member, and a separate switch is provided and is located for actuation by movement of the resilient member only during snap-action movement caused by magnetic attraction.

Lost motion is provided between the switch itself and the snap-action device which permits the switch to be so constructed as to have the most desirable contact pressure characteristics without reacting upon the characteristics or operation of the snap-action mechanism. More particularly the lost motion feature makes it possible to provide a preset minimum contact pressure between the normally closed switch contacts right up to the moment of break. Moreover, the magnet and armature system giving the snap-action can be designed for optimum operation without the limitations imposed by an integral switch. The arrangement moreover has advantages in manufacture, assembly and adjustment.

In order that the invention may more readily be understood, a preferred embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a fragmentary View in side elevation showing a switch element;

FIGURE 2 is a plan view of a portion of FIGURE 1;

FIGURE 3 is a side elevation of the snap-action mechanism with the switch omitted; and

FIGURE 4 is a perspective view of a bi-metallic thermostat in accordance with the invention, partly broken away to illustrate details of construction.

Referring firstly to FIGURES l and 2, there is shown a switch support blade 1 formed with a contact carrying tongue 2, such tongue carrying movable contacts 3 and 4 and the blade including a support tongue 5. The blade is mounted upon a support or base 6 by screwing the free end of the support tongue to a boss 8 with the movable contacts 3, 4 located between spaced fixed contacts 9 and 10 for selective engagement with one fixed contact or the other. An adjustable screw 11 engages the blade 1 at the end remote from the contacts 34 and it will be understood that variations in the position of this end of the blade as effected by the screw 11 will control the normal contact pressure between the normal contacts 3, 9. The extreme end of the blade remote from the adjustment screw 11 is designated 20.

Referring to FIGURE 3 there is there shown a permanent magnet 12 which is supported upon the base 6 by means which are not shown in detail, but so that the magnet can be selectively adjusted towards and away from the base, one mounting arrangement being described hereinafter. A U-shaped or twin armature unit com- 3,207,877 Patented Sept. 21, 1965 prising spaced leg portions 13, 14, illustrated in more detail in FIGURE 4, embraces the magnet 12 and is supported at the free end of a resilient strip 15 the other end of which is flexibly supported upon the base 6 by a spring strip 16. The ettective operating distance between the two spaced armature legs or active portions 13-14 can be varied by means of an adjustment screw 17 which has its head engaged with the armature leg 13 and which is threaded into the armature leg 14. An insulating contact actuating member 18 is secured to the free end of strip 15 and extends away therefrom in the direction of movement of the free end of strip 15 and this insulating member 18 is formed with a slot 19 for engagement with the free end 20 of the blade 1, this slot 19 being wider than the thickness of the material of the blade 1 so that there is lost motion between the blade and the slot, whereby the armature 13-14 and the actuating member 18 can move to a limited extent without any effect on the switch contacts 3, 4, 9 and 10.

FIGURE 4 illustrates the present invention as applied to a bi-metallic thermostat and to this end the resilient strip 15 is formed as a bi-metal strip, being the thermally sensitive element of the thermostat. As can be seen from FIGURE 4, the spring strip 16 is secured to a fixed mounting bracket 21 secured to the upper side of the base 6, this bracket also carrying a temperature adjusting knob 22 which has a stem 32 passing through an aperture in the strip 15 and engaging a rigid bracket member 33 located below the strip 15 and secured thereto as by riveting. Thus movement of the knob 22 relatively to the bracket 21 will adjust the position of the strip 15 and consequently the position of the armature 13-14.

The switch itself is mounted upon the base 6 below the strip 15 in the manner previously described.

As shown in FIGURE 4, the permanent magnet 12 is carried by a bridge 27 which is supported upon the base 6 by means of helical compression springs 28 and 29 housed within posts 23 and 24 and surrounding axial screws 25 and 26 which engage with the ends of the bridge 27 and are in screw-threaded engagement with a strip 30 fixedly positioned on the underside of the base 6.

During assembly, the switch blade 1 with its contacts 3 and 4 is assembled on the base 6 and the pressure between the normally closed contacts 3 and 9 is correctly set up by adjustment of the screw 11. The bi-metal strip 15 and the armature assembly is then attached to the bracket 21 with the end 20 of the blade 1 engaged in the slot 19 of the actuating member 18. The magnet 12 is now positioned centrally between armature legs 13 and 14.

The magnetic attraction between the magnet 12 and the armature is then set by adjusting the spacing between the armature legs and the magnet by the screw 17 which changes the total gap between the two armature legs. As the magnetic attractive initial force is increased by decreasing the spacing between legs 13 and 14, a greater force is necessary to move the U-shaped armature from the position in which the armature leg 13 is slightly closer to the magnet 12 to the opposite position wherein the leg 14 is closer and vice versa.

For the purpose of this description, it may be assumed that the thermostat of FIGURE 4 is arranged so that contact is made between the contacts 3 and 9 when the temperature around the thermostat is below the set temperature and that as the temperature rises above the set temperature the switch is operated to break the contacts 3 and 9 and make the contacts 4 and 10. When the temperature is below the set point, the free end of the strip 15 will be supported by the engagement of the actuating member 18 with the free end 20 of the switch blade 1 and in this position there will be a small clearance between the underside of the end 20 of the blade 1 and the adjacent upper side of the slot 19. Increasing ambient temperature will eventually generate sufiicient force in the strip 15 to overcome the magnetic bias holding the armature leg 13 towards the magnet 12 and eventually the twin armature unit and, therefore, the member 18 will begin to move so that the armature 13 recedes from the magnet while the armature 14 approaches the magnet 12. The first small part of this movement has no effect on the closed switch contact 3 and which will undergo no appreciable reduction in contact pressure until after the lost motion or clearance between the upwardly facing side of the slot 19 and the free end 21) has been taken up. As soon as the lost motion or clearance has been taken up, the switch blade 1 will be moved rapidly 'by increasing magnetic attraction between magnet 12 and up wardly moving armature leg 14 to break the contacts 3 and 9 and make the contacts 4 and 10. It should be noted that the preset minimum contact pressure established by adjusting screw 11 between the normally closed contacts 3 and 9 is maintained at its full value until the instant of snap-action.

In producing snap-action displacement of the movable contacts 3, 4 from one position to the other, the U-shaped armature passes through a neutral position wherein the forces exerted by the magnet 12 simultaneously on the legs 13, 14 are equal and opposite with no resultant force on the free end portion of the bimetallic strip 15. As the armature moves in the contact opening direction with respect to the normally closed contacts 3, 9, the lost motion provided by slot 19 will not have been taken up until after the U-shaped armature has passed through the neutral zone in the contact opening direction. In this manner, at the time the normally closed contacts are subjected to the first reduction in the minimum contact pressure provided by adjustment of screw 11, the movement of switch blade 1 in the contact opening direction will be sustained by the increasing force of attraction between the magnet 12 and the armature leg 14. This is obtained by appropriate adjustment of the magnet positioning screws 25 and the armature leg spacing screw 17.

Contacts 3 and 9 may thus be connected in a circuit which requires them to open under substantial load.

Contact pressure for the normally open contacts 4 and 10 is provided by the attraction between magnet 12 and armature leg 14 accompanied by fiexure of the tongue 2, the lost motion provided by slot 19 remaining taken up. The magnet 12 and armature legs 13, 14 are preferably so adjusted that snap action occurs before the contact pressure between the normally open contacts 4 and 10 is reduced to an objectionably low value. Alternatively, the contacts 4 and 10 may be connected in a no-load circuit such as the circuit of a locking relay so that the contacts 4, 10 are short-circuited by the relay contacts immediately upon closure. This arrangement relieves the contacts 4 and 10 of any current interrupting duty so that it is immaterial if the contact pressure is slowly reduced to zero and the contacts thereafter opened slowly prior to the snap action displacement.

It must be understood that the foregoing description of a room thermostat is given purely by way of illustration and in particular that the choice of a bi-metallic strip is illustrative.

While I have shown and described what I believe to be the best embodiments of my invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

I claim: I

1. A magnetic snap-action thermostat comprising an elongated base plate; an elongated bi-metallic strip fixedly supported by one end portion from said base plate, the other end portion being free to move in response to temperature changes; a U-shaped armature carried by the free end portion of said strip, the leg portions of said armature extending generally parallel to the longitudinal axis of said strip; an elongated flexible switch blade supported by said base plate, said switch blade being spaced from and extending generally parallel to said strip one end portion of said switch blade being free and being disposed in proximity to the free end portion of said strip, the other end portion of said switch blade being substantially fixedly positioned with respect to said base plate; an insulating finger carried by said free end portion of said strip, said finger having a slot formed therein wherein the free end portion of said switch blade is received, the width of said slot being greater than the thickness of said free end portion of the switch blade whereby a connection including lost motion is provided between the approximate free end portions of said bi-metallic strip and said switch blade; a first cont act carried by said free end portion of said switch blade; a second contact fixedly positioned with respect to said base plate for engagement by said first contact; screw adjusting means carried by said base plate and engaging said switch blade adjacent to said fixedly positioned end portion thereof for adjusting the minimum contact pressure between said cont acts with said free end portion of the said'switch blade spaced from the sides of said slot; further screw adjusting means acting between said base plate and said bi-metallic strip thereby to set the position of the free end portion of said bi-metallic strip at a predetermined temperature, a permanent magnet positioned between and spaced from the legs of said armature, said magnet being olarized to exert forces of attraction simultaneously on both of said legs thereby providing a neutral position wherein said forces are equal and opposite with no resultant force acting on said free end portion of said strip; and means carried by said base plate and supporting said magnet in a position wherein said lost motion is taken up after said armature has moved through said neutral position in the contact opening direction, movement in the contact opening direction being sustained by the increasing force of attraction between said magnet and the leg of said armature which is approaching said magnet.

2. A magnetic snap-action switch comprising a base member; a U-shaped armature carried by said first resilient member and comprising spaced leg portions embracing said permanent magnet, said leg portions being spaced from and simultaneously attracted by said magnet with equal and opposite forces in a neutral osition; a second resilient member carried by said base member; a movable contact carried by said second resilient member; at least one fixed contact fixedly positioned with respect to said base member and selectively engageable and disengageable by said movable contact; and actuator means interconnecting said first and second resilient members for simultaneous displacement of said armature and said movable contact with lost motion therebetween, the forces of attraction between said armature and said magnet varying during displacement of said armature so that the first resilient member is by said armature constrained to move through said neutral position with a snap-action in either direction between first and second limiting positions in each of which said magnet exerts a greater attractive force on one of said legs than on the other and in the first of which the movable contact engages said fixed contact, said actuator means taking up said lost motion after passage of said first resilient member through said neutral position and before the movable contact disengages said fixed contact during the course of said snap-action movement of said first resilient member from said first position to said second position.

3. A magnetic snap-action switch according to claim 2 further comprising adjustable means carried by said base member and acting on said second resilient member to vary the pressure between the contacts.

4. A magnetic snap-action switch according to claim 2 further comprising adjustable means carried by said first resilient member along with said armature and connected to said armature for varying the distance between said leg portions of said armature.

5. A magnetic snap-action switch according to claim 2 wherein said permanent magnet is supported by said base member by means including adjustment means for varying the position of the permanent magnet relative to said leg portions in the direction of said snap-action movement of said first resilient member.

6. A magnetic snap-action switch according to claim 2 wherein the first resilient member is a bi-metallic strip which displaces said armature in the direction of said snapaction movement in response to changes in the temperature of said strip.

7. A magnetic snap-action switch comprising a first resilient member at least a portion of which is movable actuation of said switch being caused by movement of said first resilient member a permanent magnet; la bifurcated armature carried by said first resilient member for movement therewith, said armature comprising leg portions spaced from and embracing said permanent magnet, said magnet being polarized to exert oppositely directed forces of attraction simultaneously on both of said leg portions; supporting means maintaining said permanent magnet positioned for relative movement between said armature and said magnet in the course of which said first resilient member and said armature move together through a neutral position in which said oppositely directed forces are equal, said neutral position lying between first and second limiting positions in each of which said magnet exerts a greater attractive force on one of said leg portions than on the other; a second resilient member at least a portion of which is movable; a first contact carried by said second resilient member for movement therewith; a second contact; means included in said supporting means maintaining said second contact positioned for selective engagement with and disengagement from said first contact during the course of movement of said second resilient member; and actuator means interconnecting said first and second resilient members for movement together with lost motion therebetween, said second resilient means yieldingly maintaining a predetermined minimum contact pressure between said first and second contacts with said first resilient means in said first limiting position, said lost motion permitting said first resilient means to move from said first limiting position toward said second limiting position independently of said second resilient means until after said first resilient means has passed through said neutral position.

8. A switch according to claim 7, wherein at least one of said resilient members is formed as an elongated strip.

9. A switch according to claim 7, wherein both of said resilient members are formed as elongated strips having spaced generally parallel longitudinal axes, both ends of each strip being located in proximity to corresponding ends of the other strip, said supporting means acting on one end portion of each strip, the other end portions being movable; and in which said actuating means comprises a member extending between said movable end portions of said strips, said member being connected to the movable portion of one of said strips and having an aperture formed therein wherein the movable portion of the other strip is received, the size of said aperture in the direction of movement of the movable portion of said other strip being greater than the corresponding size of the movable portion received therein for providing said lost motion.

References Cited by the Examiner UNITED STATES PATENTS 2,236,282 3/41 Ayers 200-122 2,272,021 2/42 Riche 200-122 2,518,966 8/50 Wilson et al 200-87 2,519,558 8/50 Flight et al. 200-67 2,550,546 4/51 Flight et al 200-67 2,601,556 6/52 Prouty 200-138 2,707,215 4/55 Chinn 200-67 2,843,720 7/58 Young 200-138 FOREIGN PATENTS 206,667 12/59 Austria.

BERNARD A. GILHEANY, Primary Examiner. 

1. A MAGNETIC SNAP-ACTION THERMOSTAT COMPRISING AN ELONGATED BASE PLATE; AN ELONGATED BI-METALLIC STRAP FIXEDLY SUPPORTED BY ONE END PORTION FROM SSAID BASE PLATE, THE OTHER END PORTION BEING FREE TO MOVE IN RESPONSE TO TEMPERATURE CHANGES; A U-SHAPED ARMATURE CARRIED BY THE FREE END PORTION OF SAID STRIP, THE LEG PORTIONS OF SAID ARMATURE EXTENDING GENERALLY PARALLEL TO THE LONGITUDINAL AXIS OF SAID STRIP; AN ELONGATED FLEXIBLE SWITCH BLADE SUPPORTED BY SAID BASE PLATE, SAID SWITCH BLADE BEING SPACED FROM AND EXTENDING GENERALLY PARALLEL TO SAID STRIP ONE END PORTION OF SAID SWITCH BLADE BEING FREE AND BEING DISPOSED IN PROXIMITY TO THE FREE END PORTION OF SAID STRIP, THE OTHER END PORTION OF SAID SWITCH BLADE BEING SUBSTANTIALY FLEXIDLY POSITIONED WITH RESPECT TO SAID BASE PLATE; AN INSULATING FINGER CARRIED BY SAID FREE END PORTION OF SAID STRIP, SAID FINGER HAVING A SLOT FORMED THEREIN WHEREIN THE FREE END PORTION OF SAID SWITCH BLADE IS RECEIVED, THE WIDTH OF SAID SLOT BEING GREATER THAN THE THICKNESS OF SAID FREE END PORTION OF THE SWITCH BLADE WHEREBY A CONNECTION INCLUDING LOST MOTION IS PROVIDED BETWEEN THE APPROXIMATE FREE END PORTIONS OF SAID BI-METALLIC STRIP AND SAID SWITCH BLADE; A FIRST CONTACT CARRIED BY SAID FREE END PORTION OF SAID SWITCH BLADE; A SECOND CONTACT FLEXEDLY POSITINED WITH RESPECT TO SAID BASE PLATE FOR ENGAGEMENT BY SAID FIRST CONTACT; SCREW ADJUSTING MEANS CARRIED BY SAID BASE PLATE AND ENGAGING SAID SWITCH BLADE ADJACENT TO SAID FIXEDLY POSITIONED END PORTION THEREOF FOR ADJUSTING THE MIMIMUM CONTACT PRESSURE BETWEEN SAID CONTACTS WITH SAID FREE END PORTION OF THE SAID SWITCH BLADE SPACED FROM THE SIDES OF SAID 